There are a variety of applications using a light source for which uniform illumination of a surface is desirable. For example, some degree of uniformity is desirable when projecting an image onto a reflective screen for viewing motion pictures. Although some of these applications do not require high uniformity of illumination of the surface, others do. For example, processes for exposing photoresists in methods of manufacturing microcircuits in which lines to be formed on a surface may be of the order of one micron apart, require a high degree of uniformity of illumination.
In general, many systems which image a light source on a plane surface to be illuminated produce uniform illumination of that surface only if there is uniform emission of light from the source. However, the angular distribution of radiation from some sources is more uniform than the spatial distribution. That is, although various points within the source have different intensities, the angular distribution of radiation from each point is substantially uniform. This fact has been taken advantage of in prior art systems as shown in FIGS. 1 and 2.
In the prior art system shown in FIG. 1, light rays emanating from source 10 pass through condenser lens 16, consisting of lens 12 and lens 14, which images the light source onto objective lens 18. Rays from lens 18 pass through collimating lens 20 and illuminate plane surface 22.
In this system, the uniformity of illumination on the image plane 22 depends on the uniformity of light coming from condenser lens set 16 which in turn is dependent upon the uniformity of angular distribution of radiation from source 10. In this system, the variation of light at the image plane 22 is greater than about 5 percent, which is inadequately uniform for some purposes.
In the prior art system shown in FIG. 2, the uniformity of light at the image plane 22 has been improved by replacing a part of the condenser lens of FIG. 1 with a composite lens 24 comprising field lenses 24a, 24b and 24c. Each of these field lenses produces an image of the source onto a corresponding objective lens 26 comprising lenses 26a, 26b and 26c, each of which in turn produces an image of its associated field lens on image plane 22. Collector lens 28 serves the function of merging the images from the objective lens into one image at plane 22.
Prior art systems, such as the one shown in FIG. 2, have been useful in producing substantially uniform light on a plane surface. However, the combination of lenses required by the system is complex and expensive, and a more simple system which provides a high uniformity of illumination on a surface is desirable.
Systems not having complex lens arrangements but instead having a focusing reflective lens have been used in the prior art to direct light rays from a light source in a desired direction. Examples of such systems are disclosed in U.S. Pat. No. 3,872,349 to Spero et al, for "Apparatus and Method for Generating Radiation". In the system disclosed in Spero et al, a microwave-generated plasma light source is disposed inside an parabolic reflector at or near the focal point of the reflector. Light rays from the source are reflected outwardly in a parallel fashion. One goal of systems such as those disclosed in U.S. Pat. No. 3,872,349 was to direct as much emitted light as possible at an object being irradiated without regard to the uniformity of the illumination falling on the object. Since it was well-known that a light source at the focal point of a focusing reflector, such as an elliptical or a parabolic reflector, will not uniformly illuminate a surface, the potential of the combination of a light source and a focusing reflector in providing uniform illumination on a surface was not recognized in the prior art.
The invention disclosed herein is based on the unexpected discovery that light rays emitted from a volume source at or near the focal point of a focusing reflector are reflected in pathways which combine in such a manner as to provide uniform illumination of a surface, and furthermore, that the illumination on the surface is highly uniform even when the source itself is non-uniform.